The increasing commercial importance of rechargeable lithium ion battery cells has prompted a desire to identify and to prepare cathode materials better able to reversibly intercalate and deintercalate lithium ions at greater voltages. There are three prominent reversible lithium intercalation compounds used for lithium ion rechargeable batteries: lithium cobalt oxide (LiCoO.sub.2) and lithium nickel oxide (LiNiO.sub.2) compounds, as well as lithium magnesese oxide (LiMn.sub.2 O.sub.4) spinel.
LiCoO.sub.2 cells are of particular interest because of their ability to insert/deinsert lithium reversibly at voltages greater than 4 V resulting in batteries that have an output voltage and an energy density 3 times greater than Ni--Cd. The theoretical charge capacity of LiCoO.sub.2 cells is large at about 275 Amp-hours/kilogram (A-h/kg). In practical application, however, the maximum obtainable capacity for LiCoO.sub.2 cells has been only about 140 A-h/kg, corresponding to a maximum charge voltage of about 4.2 V.
Previous attempts to exceed this charge cutoff voltage in LiCoO.sub.2 cells have caused poor cell performance manifested by severe loss of charge capacity in subsequent charge-discharge cycles. The commonly-held reason for the 4.2 volt charge limitation for LiCoO.sub.2 cells was that electrochemical delithiation of LiCoO.sub.2 above this voltage destabilized the structure of the partially delithiated LiCoO.sub.2 phase, impairing intercalation of lithium in subsequent charge-discharge cycles.
Lithium cobalt oxide adopts a hexagonal structure consisting of CoO.sub.2 layers separated by a Van der Waals gap. The octahedral sites within the Van der Waals gap are occupied by the Li.sup.+ ions. This results in the reversible intercalation of lithium.
In such compounds, lithium acts as a glue or cement, screening the repulsive interactions between the negatively charged CoO.sub.2 layers. When the compound is fully lithiated LiCoO.sub.2, the screening effect is greatest. As lithium is removed, the screening effect is decreased and the repulsions between the two CoO.sub.2 layers are enhanced resulting in an expansion of the c-axis parameter. Due to the screening effect of lithium, it was believed that complete lithium deintercalation to form CoO.sub.2 was not possible.
Ohzuku et al., J. Electrochem. Soc., Vol. 141, No. 11, Nov. 1994, p. 2972, have succeeded in removing approximately 85% of the lithium. Their efforts revealed a monoclinic phase and questioned the existence of a CoO.sub.2 phase.
In another theory, Reimers and Dahn, J. Electrochem. Soc., 139, 2091, (1992), states that excess Co.sup.4+ destroys the crystallinity of the lithium cobalt oxide structure. Apparently, it inhibits the formation of highly crystalline phases at low lithium contents.
Wizansky, Rauch, and DiSalvo, Journal of Solid State Chemistry, 81, 203-207 (1989), investigated the delithiation of LiCOO.sub.2 through the use of powerful oxidizing agents such as NO.sub.2.sup.+ and MoF.sub.6. Their results showed that this approach merely decomposes the LiCoO.sub.2.
LiNiO.sub.2 is isostructural with LiCoO.sub.2 and is commercially viable for use in secondary lithium ion batteries. Heretofore, no one has been capable of obtaining the delithiated NiO.sub.2 phase. Ohzuku et al., J. Electrochem. Soc., Vol. 140, No. 7, July 1993, working with the nickel oxide reported Li.sub.0.06 NiO.sub.2 and approximated that this was the end phase.
Lithium secondary batteries are generally recognized and are described for instance in U.S. Pat. No. 5,296,318 to Gozdz et. al., which is incorporated in its entirety herein by reference. Lithium metal-free "rocking-chair" batteries may thus be viewed as comprising two lithium-ion-absorbing electrode "sponges" separated by a lithium-ion-conducting electrolyte, usually comprising a Li.sup.+ salt dissolved in a non-aqueous solvent or mixture of such solvents. Numerous such salts and solvents are known in the arts, as evidence in Canadian Pat. Publication No. 2,022,191, dated 30 Jan. 1991.
When cells comprising these previously-available electrolytes are cycled to a voltage even slightly greater than 4.3 V, electrolyte oxidation occurs. Although small, this oxidation can jeopardize the capacity, cycle life, and safety of the battery cell. For example, the electrode oxidation reaction consumes part of the charging current, which cannot be recovered when discharging the cell. The result is a continuous loss in the cell capacity over subsequent cycles. Further, if during each charge a small part of the electrolyte is consumed, excess electrolyte must be included when the cell is assembled. The excess electrolyte reduces the amount of active material for a constant volume battery body, thereby decreasing initial capacity. In addition, the oxidation of the electrolyte often generates solid and gaseous by-products. The solid by-products build up a passivating layer on the particles of the active material, essentially increasing the polarization of the cell and lowering the output voltage. Simultaneously, and more importantly, the gaseous by-products increase the internal pressure of the cell, thereby increasing the risk of explosion and leading to unsafe and unacceptable operating conditions.
U.S. Pat. No. 5,192,629, which is herein incorporated by reference in its entirety, provides a class of electrolyte compositions that are exceptionally useful for minimizing electrolyte decomposition in secondary batteries comprising strongly oxidizing positive electrode materials. These electrolytes are thus uniquely capable of enhancing the cycle life and improving the temperature performance of practical "rocking chair" cells. These electrolyte compositions have a range of effective stability extending up to about 5.0 V at 55.degree. C., as well as at room temperature (about 25.degree. C.).
Electrolytes that are substantially inert to oxidation include a 0.5M to 2M solution of LiPF.sub.6, or LiPF.sub.6 with up to about an equal amount of LiBF.sub.4 added, in a mixture of dimethylcarbonate (DMC) and ethylene carbonate (EC) within the weight percent ratio range from about 95 DMC:5 EC to 20 DMC:80 EC. In a preferred electrolyte solution, the solvent ratio range is about 80 DMC:20 EC to 20 DMC:80 EC. An optimum composition for operation at room temperature and below is an approximately 1.5M LiPF.sub.6 solution in a solvent mixture of about 67 DMC:33 EC. A battery operating at room temperature and higher, e.g., in the range of 55.degree. C., optimally utilizes an electrolyte consisting essentially of an approximately 1.5M LiPF.sub.6 solution in a solvent combination of about 33 DMC:67 EC. An additionally useful electrolyte consists essentially of an approximately 1M to 2M solution of equal pares of LiPF.sub.6 and LiBF.sub.4 in a solvent mixture of about 50 DMC:50 EC.
Negligible current increases, after the reversible Li intercalations, at voltages up to about 5 V vs. Li indicates this remarkable stability that enables enhanced cell capacity not only in the "rocking chair" cells comprising negative electrodes of carbon, e.g., petroleum coke, but also in Li negative electrode cells. Such a lithium metal cell utilizing a LiCoO.sub.2 positive electrode may be reasonably expected to achieve normal operating ranges of about 4.3 to 5.1 V.
With the aid of electrolytes which are substantially inert to oxidation and solid state electrolytic cells, fully delithiated phases of both CoO.sub.2 and NiO.sub.2 were obtained.